In recent years, as high-speed wireless access systems using a band of 2.4 GHz or 5 GHz, the spread of systems of the Institute of Electrical and Electronics Engineers (IEEE) 802.11g standard, the IEEE 802.11a standard, and so on has been remarkable. In these systems, a physical layer transmission speed of up to 54 Mbps is achieved using an orthogonal frequency division multiplexing (OFDM) modulation scheme, which is technology used to stabilize the performance in a multipath fading environment.
However, the transmission speed referred to here is a transmission speed on a physical layer. Because the transmission efficiency of a medium access control (MAC) layer is actually about 50% to 70%, an upper limit of an actual throughput is about 30 Mbps and its performance is further degraded if the number of communication partners requiring information increases. On the other hand, in the field of wired local area networks (LANs), the provision of a high-speed link of 100 Mbps is widespread as a result of the spread of fiber to the home (FTTH) using optical fibers in individual homes such as a 100 Base-T interface of Ethernet (registered trademark). A further increase in a transmission speed is also required in the field of the wireless LANs.
As technology therefor, multiple-input multiple-output (MIMO) technology is introduced into IEEE 802.11n as spatial multiplexing transmission technology and up to four-element antennas are optionally supported. Furthermore, in IEEE 802.11 ac, a multiuser (MU)-MIMO communication method is studied and the number of antenna elements to be supported is increased to 8 (Non-Patent Document 1). There is a problem in that the number of pilot signals for channel estimation increases when the number of antenna elements increases.
FIG. 6 illustrates a configuration of a conventional transmission/reception system. In FIG. 6, 1 denotes an access point, 2-1 to 2-i denote a plurality of stations, 11 denotes a data selection/output circuit, 12 and 24-1 to 24-i denote transmission signal generation circuits, 13 and 22-1 to 22-i denote wireless signal transmission/reception circuits, 141 to 14N, 211-1 to 21M-1, and 211-i to 21M-i denote transmit/receive antennas, 15 and 23-1 to 23-i denote reception signal demodulation circuits, 16 denotes a channel information storage circuit, 17 denotes a transmission method determination circuit, and 18 denotes a channel estimation symbol generation circuit. Here, 1≦i≦K, where K is the number of the stations, M, is the number of the transmit/receive antennas of an ith station, and N is the number of the transmit/receive antennas of the access point. In addition, in consideration of an OFDM system, the number of frequency channels (subcarriers) carrying signals is assumed to be F.
At the transmission from the access point 1 to a station 2-i, the access point 1 collects channel information representing information about a propagation environment with the antennas of each station before the access point 1 transmits data. The channel estimation symbol generation circuit 18 generates a training symbol used to estimate the channel information with the antennas of the station and outputs it to the transmission signal generation circuit 12. The transmission signal generation circuit 12 attaches a guard interval, a control signal, and the like. The wireless signal transmission/reception circuit 13 up-converts an output signal of the transmission signal generation circuit 12 into a carrier frequency and performs transmission via the transmit/receive antennas 141 to 14N.
In the station 2-i, the wireless signal transmission/reception circuit 22-i receives a wireless signal via the transmit/receive antenna 21M-i, and the reception signal demodulation circuit 23-i performs synchronization, channel estimation, and demodulation and outputs estimated channel information or channel-related information generated by, for example, compressing the channel information to the transmission signal generation circuit 24-i. The transmission signal generation circuit 24-i generates a transmission signal including the channel information or the channel-related information, and outputs it to the wireless signal transmission/reception circuit 22-i at a predetermined timing. The wireless signal transmission/reception circuit 22-i transmits a signal including the input channel information or the channel-related information via at least one of the transmit/receive antennas 211-i to 21M-i.
In the access point 1, a signal from the station 2-i is input to the wireless signal transmission/reception circuit 13 via at least one of the transmit/receive antennas 141 to 14N. The wireless signal transmission/reception circuit 13 performs conversion into digital data by down-converting the input signal from the station 2-i and outputs it to the reception signal demodulation circuit 15. The reception signal demodulation circuit 15 demodulates a reception signal, extracts channel information included in the reception signal, and outputs it to the channel information storage circuit 16.
When the channel information of the station 2-i serving as a communication partner is stored, the data selection/output circuit 11 outputs information on the determined communication partner to the transmission method determination circuit 17 and outputs data to the transmission signal generation circuit 12. The transmission method determination circuit 17 determines transmission weights from the channel information stored in the channel information storage circuit 16, and outputs a transmission mode including the number of spatial streams, a modulation scheme, and a coding rate as well as the transmission weights to the transmission signal generation circuit 12. The channel estimation symbol generation circuit 18 generates a training symbol used to estimate channel information with the antennas of the station and outputs it to the transmission signal generation circuit 12. In accordance with the transmission mode determined for the station which performs communication, the transmission signal generation circuit 12 modulates and encodes a transmission signal, multiplies the transmission weights, inserts a pilot signal to be used in signal detection and communication information delivery, and outputs to the wireless signal transmission/reception circuit 13. The wireless signal transmission/reception circuit 13 up-converts an input signal into a carrier frequency and transmits via the transmit/receive antennas 141 to 14N.
As a method for selecting the communication partner, a communication partner which saves data to be transmitted in a memory and which is ready to perform transmission may be designated, a communication partner corresponding to the oldest data among a plurality of pieces of saved data may be selected, a communication partner may be selected based on quality of service (QoS) of a user, a combination of users previously determined by a group identifier (ID) may be selected, or a combination of communication partners having a low correlation of channel information may be selected.
Hereinafter, a block diagonalization (BD) directivity control method will be described as an example in which communication with a plurality of communication partners is performed using the spatial multiplexing scheme. In order to acquire channel information for the antennas of the stations 2-1 to 2-K, in the access point 1, the channel estimation symbol generation circuit 18 generates a training symbol used for channel estimation. A configuration example of the training symbol is illustrated in FIG. 7. Rectangular blocks LT (1, 1) to LT (N, N) illustrated in FIG. 7 represent OFDM symbols for the channel estimation, and guard intervals (GIs) are attached thereto. LT (j, k) corresponds to an OFDM symbol to be transmitted from a jth antenna at a kth timing in a training symbol block. LT (j, k) is obtained by performing an inverse Fourier transform on signals using known signals s1 to sF at frequency channels and attaching a GI thereto. A transmission signal matrix X including transmission signals xj, k, n of nth frequency channels of LT (1, 1) to LT (N, N) is represented as follows:
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                                                                X                n                            =                            ⁢                              (                                                                                                    x                                                  1                          ,                          1                          ,                          n                                                                                                                                    x                                                  1                          ,                          2                          ,                          n                                                                                                            …                                                                                      x                                                  1                          ,                          N                          ,                          n                                                                                                                                                                        x                                                  2                          ,                          1                          ,                          n                                                                                                                                    x                                                  2                          ,                          2                          ,                          n                                                                                                            …                                                                                      x                                                  2                          ,                          N                          ,                          n                                                                                                                                                ⋮                                                              ⋮                                                              ⋱                                                              ⋮                                                                                                                          x                                                  N                          ,                          1                          ,                          n                                                                                                                                    x                                                  N                          ,                          2                          ,                          n                                                                                                            …                                                                                      x                                                  N                          ,                          N                          ,                          n                                                                                                                    )                                                                                        =                            ⁢                                                (                                                                                                              a                                                      1                            ,                            1                                                                                                                                                a                                                      1                            ,                            2                                                                                                                      …                                                                                              a                                                      1                            ,                            N                                                                                                                                                                                        a                                                      2                            ,                            1                                                                                                                                                a                                                      2                            ,                            2                                                                                                                      …                                                                                              a                                                      2                            ,                            N                                                                                                                                                              ⋮                                                                    ⋮                                                                    ⋱                                                                    ⋮                                                                                                                                      a                                                      N                            ,                            1                                                                                                                                                a                                                      N                            ,                            2                                                                                                                      …                                                                                              a                                                      N                            ,                            N                                                                                                                                )                                ⁢                                  s                  n                                                                                                        =                            ⁢                              As                n                                                                        (        1        )            
Here, xj, k, n represents a transmission signal corresponding to an nth frequency channel of LT (j, k), A is a code matrix for the training symbol and a known matrix is used by transmission and reception ends. If an identity matrix I is used as A, it is possible to transmit sn from each antenna at a different timing. Alternatively, an orthogonal matrix that satisfies AHA=I can be used as A. Here, the superscript H represents an Hermitian transpose (complex conjugate transpose).
In the station 2-i, when a signal from the access point 1 is received, the reception signal demodulation circuit 23-i performs synchronization on the received signal and estimates a channel matrix including propagation coefficients between the access point 1 and the antennas of the station 2-i, which is channel information, from reception signals corresponding to LT (1, 1) to LT (N, N). In a reception signal corresponding to a jth training symbol, a reception signal vector yj, i, n including reception signals yl, j, i, n to yMi, j, i, n received by the receive antennas 211-i to 21M-i on an nth frequency channel is represented as follows:
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                                                                (                                                                                                    y                                                  1                          ,                          j                          ,                          i                          ,                          n                                                                                                                                                                        y                                                  2                          ,                          j                          ,                          i                          ,                          n                                                                                                                                                ⋮                                                                                                                          y                                                                              M                            i                                                    ,                          j                          ,                          i                          ,                          n                                                                                                                    )                            =                            ⁢                              y                                  j                  ,                  i                  ,                  n                                                                                                        =                            ⁢                                                                    H                                          i                      ,                      n                                                        ⁡                                      (                                                                                                                        x                                                          1                              ,                              j                              ,                              n                                                                                                                                                                                                        x                                                          2                              ,                              j                              ,                              n                                                                                                                                                                            ⋮                                                                                                                                                  x                                                          N                              ,                              j                              ,                              n                                                                                                                                            )                                                  +                                  n                                      j                    ,                    i                    ,                    n                                                                                                          (        2        )            
Here, Hi, n is a channel matrix ((Mi×N) matrix) representing channel information about the nth frequency channel of the station 2-i. An element of a pth column and a qth row of the channel matrix Hi, n represents a propagation coefficient between a pth transmit antenna of the access point 1 and a qth receive antenna of the station 2-i. nj, i, n represents a thermal noise vector on the transmit/receive antennas 211-i to 21M-i of the nth frequency channel of the station 2-i at a reception timing of the jth training symbol.
Consequently, in the station 2-i, a reception signal matrix Yi, n corresponding to first to Nth training symbols on an nth frequency channel is represented as follows:
[Formula 3]Yi,n=(yl,i,n . . . yN,i,n)=Hi,nXn+Ni,n  (3)Ni, n represents a thermal noise matrix for signals received by the transmit/receive antennas 211-i to 21M-i at reception timings of the first to Nth training symbols on the nth frequency channel of the station 2-i. 
Because A and sn are known in the station 2-i, Xn is known and the channel matrix can be obtained from the reception signal matrix Yi, n as follows:
[Formula 4]Hi,n′=Yi,nXn−1=Hi,n+Ni,nXn−1  (4)The station 2-i may directly set the obtained H′i, n as feedback information or it may feed back Li row vectors of H′i, n (Li is the number of channel dimensions necessary to perform transmission to the station 2-i and is an integer greater than or equal to the number of data streams Li′ for the station 2-i).
In addition, singular value decomposition on H′i, n may be performed, Li vectors may be selected from a right singular matrix V(s)i, n ((N×Mi) matrix) corresponding to a signal space obtained as the following equation, and they may be fed back.
[Formula 5]Hi,n′=Ui,nΣi,n(Vi,n(s)Vi,n(n))H  (5)Here, Ui, n is a left singular matrix, Σi, n is an (Mi×N) diagonal matrix in which a singular value is set as a diagonal element and a non-diagonal term is 0, and V(n)i, n((N×(N−Mi)) matrix) represents a collection of vectors that do not correspond to singular values in the right singular matrix. Here, although the number of the antennas N of the access point 1 is assumed to be greater than the number of the antennas Mi of the station 2-i, when the number of the antennas Mi of the station 2-i is greater than the number of the antennas N of the access point 1, all column vectors of the right singular matrix correspond to singular values, and Li vectors of the right singular matrix V(s)i, n((N×N) matrix) may be selected and fed back.
Alternatively, Li vectors among basis vectors obtained by performing a Gram-Schmidt orthogonalization method on row vectors of an Hermitian matrix of H′i, n may be selected and fed back. The basis vectors obtained by the orthogonalization method can also be obtained by QR decomposition, the QR decomposition may be performed as the following equation, and Q(s)i, n corresponding to an (Mi×Mi) triangular matrix Γi, n between the obtained orthogonal matrices (Q(s)i, n and Q(n)i, n) may be fed back.
                    [                  Formula          ⁢                                          ⁢          6                ]                                                                                  (                          H                              i                ,                n                            ′                        )                    H                =                              (                                                                                Q                                          i                      ,                      n                                                              (                      s                      )                                                                                                            Q                                          i                      ,                      n                                                              (                      n                      )                                                                                            )                    ⁢                      (                                                                                Γ                                          i                      ,                      n                                                                                                                    0                                                      )                                              (        6        )            Here, Li is the number of channel dimensions, the feedback of which has been requested from the access point 1 to the station 2-i, and the number of spatial streams L′, in which the access point 1 actually performs transmission to the station 2-i is set to be less than or equal to Li. Q(s)i, n and V(s)i, n are part of a unitary matrix, and these matrices obtained from the channel information are defined as signal space matrices or signal space information.
The stations 2-1 to 2-K inserts, into a transmission signal, information about the Li vectors or information compressed in the form in which the information about the Li vectors is reproducible using a feature of the unitary matrix and the like, and transmit it to the access point 1 via the wireless signal transmission/reception circuit 22-i. The access point 1 acquires, from the stations 2-1 to 2-K, the channel information or signal space information calculated from the channel information and stores it in the channel information storage circuit 16.
In the access point 1, when transmission to the stations 2-1 to 2-K is performed, the transmission method determination circuit 17 acquires fed back information about the Li vectors from the channel information storage circuit 16. An (N×Li) matrix including the Li vectors obtained for an nth frequency channel for the station 2-i is defined as a signal space matrix Gi, n. Gi, n may be a complex conjugate transpose matrix of Li row vectors of an estimated channel matrix, or a reception weight matrix Ri, n for Li data streams may be previously defined in the station 2-i and Gi, n may be set as follows:
[Formula 7]Gi,n=(Ri,nHi,n)H  (7)
Alternatively, it may be set as follows:
                    [                  Formula          ⁢                                          ⁢          8                ]                                                                      G                      i            ,            n                          =                              [                          V                              i                ,                n                                            (                s                )                                      ]                                L            i                                              (        8        )                                [                  Formula          ⁢                                          ⁢          9                ]                                                                      G                      i            ,            n                          =                              [                          Q                              i                ,                n                                            (                s                )                                      ]                                L            i                                              (        9        )            [A]L is a function of obtaining an (N×L) matrix by selecting L column vectors from an (N×M) matrix A. When Li is a number less than or equal to Mi and Gi, n is defined by Equation (7), if Li<Mi, the station 2-i may calculate a reception weight Ri, n for Gi, n in the station 2-i in advance and store it. For the reception weight Ri, n in the station 2-i, it is possible to use a reception weight obtained using a matrix obtained by multiplying Hi, nH from the right of Hi, n an Hermitian matrix of an eigenvector of a matrix obtained by multiplying Hi, nH from the right of Hi, n, and so on. In addition, when V(s)i, n and Q(s)i, n of Equations (8) and (9) are obtained, V(s)i, n and Q(s)i, n calculated using a channel matrix Ri, nH′i, n modified using the reception weight Ri, n instead of H′i, n in Equations (5) and (6) may be used.
Next, a method for determining transmission weights using signal space information will be described with respect to an example of the BD method among multiuser transmission methods. Here, a method for calculating transmission weights for an ith station 2-i when communication is performed for K users (the stations 2-1 to 2-K) will be described. First, a set signal space matrix G+i, n corresponding to stations other than the station 2-i is defined as follows:
                    [                  Formula          ⁢                                          ⁢          10                ]                                                                      G                      i            ,            n                    +                =                  (                                                                      G                                      1                    ,                    n                                    H                                                                                    ⋮                                                                                      G                                                            i                      -                      1                                        ,                    n                                    H                                                                                                      G                                                            i                      +                      1                                        ,                    n                                    H                                                                                    ⋮                                                                                      G                                      K                    ,                    n                                    H                                                              )                                    (        10        )            
Singular value decomposition on G+i, j can be represented as follows:
[Formula 11]Gi,n+=Ui,n+Σi,n+(Vi,n(s)+Vi,n(n)+)H  (11)V(s)+i, n is a signal space vector corresponding to an eigenvalue Σ+i, n, and V(n)+i, n is a null space vector that has no eigenvalue or it is a null space vector corresponding to an eigenvalue 0. Here, when communication is performed for a null space represented by V(n)+i, n, no interference is generated with respect to reception weights of communication partners other than the station 2-i. Consequently, when communication is performed using a spatial multiplexing scheme for a plurality of communication partners, weights obtained by performing a linear operation on V(n)+i, n obtained here can be used as transmission weights to be used in the nth frequency channel.
For example, it is possible to set, as the transmission weights, weights obtained by multiplying V(n)+i, n by an Hermitian matrix of a basis vector obtained by performing the orthogonalization method on a row vector of Gi, nHV(n)+i, n obtained by multiplying the Hermitian matrix Gi, nH of the signal space matrix corresponding to the station 2-i by V(n)+i, n or by a right singular vector obtained by performing singular value decomposition on Gi, nHV(n)+i, n. When the matrix obtained from Gi, nHV(n)+i, n is denoted as Di, n, the transmission weights Wi, n are represented by [V(n)+i, nDi, n]L′i.
Although this is an MU-MIMO transmission method based on the BD method, it is possible to calculate Wi, n from a signal space matrix using a zero forcing method, a minimum mean square error (MMSE) method, a successive optimization method, Tomlinson Harashima precoding, dirty paper coding, or the like.
In addition, when the number of users is 1 (K=1), it is possible to set the signal space matrix Gi, n as transmission weights or set a right singular matrix corresponding to a signal space of the signal space matrix Gi, n as transmission weights.
In this manner, transmission weights for each communication partner can be calculated and the obtained transmission weights Wn for an nth frequency channel for K users can be represented as follows:
[Formula 12]Wn=(W1,nW2,n . . . WK,n)  (12)
By performing decoding calculation by the reception end using the transmission weights, interference between users is canceled, the interference has a form that can be eliminated by the reception end, or the interference is reduced to a certain expected value in any MU-MIMO transmission weight determination such as a BD method, a zero forcing method, an MMSE method, a successive optimization method, Tomlinson Harashima precoding, or dirty paper coding (for example, see Non Patent Document 1).